The didemnins, marine organism-derived cyclic depsipeptides, were isolated from the Caribbean tunicate Trididemnum solidum by Rinehart et al. as antitumor and antiviral reagents.sup.3-5 and numerous biological studies on didemnins have been conducted..sup.6,7 In in vitro studies, didemnin B (DB, shown below as Compound 1, hereafter, 1), one of the most potent components, was cytotoxic to L1210 murine leukemia cells at very low concentrations, and to CV-1 monkey kidney cells at much higher concentrations..sup.3 ##STR1##
In in vivo studies, 1 was effective in P388 and B16 murine tumor models and in a Yoshida ascites model..sup.3,8 Compound 1 was the first marine-derived compound to be evaluated in Phase I and Phase II clinical trials by the National Cancer Institute. As an antitumor agent.sup.9 it has shown complete or partial response in previously treated non-Hodgkins lymphomas..sup.10 Inhibition of protein synthesis and, to a lesser extent, DNA synthesis were earlier proposed as the mode of action for the cytotoxicity of the didemnins..sup.11a Recently, Crews, et al. reported purification of a didemnin-binding protein from a bovine brain homogenate which appeared to be identical to human translational elongation factor-1.alpha. (EF-1.alpha.) by affinity chromatography using N-biotinylbis(.epsilon.-aminocaproyl)didemnin A as a ligand. Didemnin A binds to EF-1.alpha. in a GTP-dependent manner, i.e., it binds to the GTP-EF-1.alpha. complex, but does not inhibit EF-1.alpha.'s GTPase activity..sup.11a More recently, SirDeshpande and Toogood have reported that didemnin B inhibits protein synthesis by stabilizing aminoacyl-tRNA binding to the ribosomal A-site, preventing translocation but not peptide bond formation..sup.11a Most recently, didemnin B has been reported to induce apoptosis in human HL-60) cells at the most rapid rate yet recorded..sup.11d
Both didemnins A (shown above as Compound 2, hereafter, DA, or 2) and B (1) have been shown to be strong inhibitors of various viruses in vitro..sup.3,12 Compounds 1 and 2 also showed activity in vitro against strains of the lethal RNA viruses Venezuelan equine encephalomyelitis, yellow fever, sandfly fever, Rift Valley fever and a Pichinde virus, for all of which no effective chemotherapeutic agents exist..sup.13 In vivo, 1 protected 90% of Rift Valley fever virus-infected mice, although considerable toxicity to the host animal was observed..sup.5,13
In vivo testing, of 1 and 2 against herpes simplex virus-2 (HSV-2) in mice has shown some efficacy in topical administration, but intracranial administration against encephalitis HSV-1, subcutaneous administration against Semliki-Forest virus, and cutaneous application against HSV-1 failed because of narrow therapeutic indexes..sup.5,12 On the whole, the high toxicity of 1 and 2 precludes their use as antiviral agents.
Didemnin B (1) inhibited lymphocyte blastogenesis and the mixed lymphocyte reaction (MLR) in vitro in murine cells,.sup.14 requiring lower concentrations than cyclosporin A tested in a human lymphocyte system..sup.15 Some efficacy was observed in the graft-vs-host (GVH) reaction in mice and allograft transplantation in a rat with an auxiliary heart graft after treatment with..sup.14,16
While 1 has shown a remarkable spectrum of biological activities, each has been accompanied by considerable toxicity.
Hence, modifications of the compound to increase a specific bioactivity while attenuating its general toxicity is a worthy endeavor.
Rinehart et al. have previously reported some structural modifications and structure-activity relationships (SAR's) of didemnins..sup.7 These preliminary results revealed that simple acylation of the N-terminus of didemnin A enhanced activities dramatically..sup.3c,7 Several limited SAR studies of didemnins have been recently reported by others. Jouin and co-workers reported a total synthesis of nordidemnin B and four nordidemnin congeners and their in vitro and in vivo activities: mandelyl-Pro-,(p-hydroxyphenyl)propionyl-Pro-, and palmityl-Pro-nordidemnin A, and [L-MeLeu.sup.7 ] nordidemnin B..sup.17 Kessler and co-workers reported the preparation and solution structure of [L-MeLeu.sup.7 ]-didemnin B..sup.18 Most recently Joullie and co-workers reported synthesis and bioactivity of didemnin B congeners: [dehydro-L-Pro.sup.8 ] and [trans-4-hydroxyPro.sup.8 ] DB, and dehydro-L-Pro-DA and trans-4-hydroxyPro-DA as well as L-Pro-DA..sup.19 The biological results can be summarized as follows: mandelyl-Pro-norDA, (p-hydroxyphenyl)-propionyl-Pro-nor DA, and [dehydroPro).sup.8 ] analogues showed comparable activity to 1, but trans-4-hydroxyPro.sup.8 analogues showed somewhat weaker cytotoxicity than that of DB. Palmityl-Pro-norDA and [L-MeLeu.sup.7 ] congeners had significantly diminished activity in the inhibition of tumor cell growth. The [dehydroPro.sup.8 ] analogues showed comparable antiproliferative activity in an in vitro immunosuppressive assay..sup.19
Numerous natural and semi-synthetic didemnin compounds have been reported in patents. See for example, U.S. Pat. No. 4,948,791; U.S. Pat. No. 5,137,870; U.S. Pat. No. 5,294,603; U.S. Pat. No. 4,782,135; U.S. Pat. No. 4,493,796; U.S. Pat. No. 4,548,814; U.S. Pat. No. 4,950,649; the disclosures of which are hereby incorporated herein by reference.
The term "immunomodulation" denotes an intended physical or chemical alteration in the function of a host's immune system. The distinction is made between manipulations which cause a decrease and others which cause an increase in number and/or function of immune mechanisms. The desire and necessity to artificially control immune responses were born during the advent of clinical transplantations in the years 1 965-75 when the immunologists were faced with the daunting task of preventing graft rejections. Hence was born the era of "Biological Response Modifiers (BRM's)" which can be classed as either immunosuppressive or immunostimulatory. A special subgroup of the latter exerts its stimulatory effects only on defective immune mechanisms and directs them towards normalization. Such substances are termed "immune restorative agents".
In the past, living and attenuated micro-organisms, autologous and heterologous proteins and injections of animal organ preparations were used with the aim of restoring an impaired defense mechanism. At present, thymic peptides, synthetic low molecular weight compounds, chemically modified nucleotides, polysaccharides from fungi and, especially, some plant extracts are also used for the same purpose.
The recently renewed interest in the immunomodulators arose from their potential therapeutic use in several clinical situations including: (a) chronic bacterial and/or viral infections; (b) immune dysfunction; (c) immune deficiency syndromes; and (d) tumors. Although an extensive search for plant derived natural products with immunostimulatory/immunosuppressive activities is currently under way, very few such substances have entered the market.